The present invention relates to a novel lithographic double-coated patterning plate with an undercoat levelling layer and a method for forming a patterned resist layer therewith. More particularly, the invention relates to a lithographic patterning plate having, on a substrate, an undercoat levelling layer formed from a novel resinous material and a layer of a photosensitive resin composition thereon as well as to an efficient method for forming a patterned resist layer therewith.
In the technology in recent years for the manufacture of various kinds of, for example, semiconductor devices by utilizing the photolithographic patterning process, it is a remarkable trend that the degree of fineness and accuracy of patterning required is increasing year by year in order to achieve a higher and higher density of integration for which the patterning works must be performed on a surface sometimes having a stepwise level difference as a consequence of a multilayered circuit pattern. When a photolithographic resist layer is formed on such a surface having a stepwise level difference, accordingly, it is unavoidable that the surface of the resist layer is also microscopically not flat enough to greatly affect the quality of the patterned resist layer.
One of the methods disclosed in Japanese Patent Kokai 3-180033 to solve the above described problem is to use a lithographic double-layered patterning plate which is prepared by first forming an undercoating layer of a thermoplastic or thermally flowable synthetic resin such as poly(methyl methacrylate) on the surface of a substrate such as a semiconductor silicon wafer and, after heating of the undercoat layer to effect levelling of the undercoat layer imparted with perfect flatness of the surface, then forming a uniform layer of a photosensitive resist composition on the flat surface of the undercoat layer. In the use of such a double-coated patterning plate, the upper coating layer of the photosensitive resist composition is first patterned by a photolithographic method known per se to give a high-quality patterned layer which can then be used, after silylation to increase the chemical stability, as a masking in the patterning of the undercoat layer by the method of dry etching, e.g., reactive ion etching method by using oxygen gas.
This method has several disadvantages. For example, intermixing of the resinous compositions sometimes takes place between the undercoat layer formed from a poly(methyl methacrylate) and the overcoat layer formed, for example, from a novolac resin-based photosensitive resist composition unavoidably resulting in a decrease in the resolution of the pattern. Moreover, the process of silylation is very troublesome because the treatment is performed by the plasma treatment method in an atmosphere of the vapor of a silicon-containing compound as a silylating agent such as silicon tetrafluoride and silicon tetrachloride.
An alternative method disclosed in Japanese Patent Kokai 62-258449 also utilizes a double-coated patterning plate prepared by forming an undercoat photoresist layer and an overcoat photoresist layer having different sensitivities against actinic rays successively on the surface of a substrate. In this case, a first photolithographic process is undertaken to effect patterning of the overcoat layer alone by the pattern-wise exposure to light followed by a wet-process development treatment and a silylation treatment and then the undercoat layer is patterned with the patterned overcoat layer as a masking by the exposure to far-ultraviolet light followed by a development treatment.
This method, in which the undercoat layer is formed, usually, from a ternary copolymeric resin consisting of methyl methacrylate, methacrylic acid and methacrylic acid anhydride in view of the photosensitivity of the resin in the far-ultraviolet region, is also disadvantageous due to the troublesomeness of the patterning process including irradiation of the undercoat layer with far-ultraviolet light in addition to the relatively low selectivity ratio of the pattern. The selectivity ratio here implied is a ratio of the decrease in the thickness of the undercoat layer when the layer is subjected to an etching treatment to the decrease in the thickness of the overcoat layer after silylation when it is subjected to the same etching treatment as for the undercoat layer. It is desirable to have a high selectivity ratio which means that the undercoat layer can be completely removed by an etching treatment while the overcoat layer is little susceptible to the same etching treatment to retain the thickness before the etching treatment.
Japanese Patent Kokai 3-114217 and 2-53058 disclose a low-reflectance coating layer, which is used in the same purpose as in the present invention, consisting of a poly(methyl methacrylate) or poly(glycidyl methacrylate) with admixture of a dye capable of absorbing ultraviolet light. The low-reflectance coating layer using a poly(methyl methacrylate), however, has a problem that intermixing between layers is sometimes unavoidable while the layer formed by intermixing is insoluble in an alkaline aqueous solution so that the cross section of the patterned resist layer obtained therewith has trailing skirts in the profile. The coating layer by using a poly(glycidyl methacrylate) as the resinous ingredient, on the other hand, has excessively high resistance against dry etching so that the dry etching must be performed so extensively that the upper resist layer is also attacked thereby to cause a decrease in the thickness of the layer consequently resulting in rounded shoulder portions in the cross sectional profile of the patterned resist layer. Further, Japanese Patent Publication 3-81122 discloses a color filter formed by providing, on the surface of a substrate, a colored layer containing an organic coloring matter and a resinous coating layer consisting of a poly(glycidyl methacrylate) containing a benzophenone-based compound as an ultraviolet absorber. The coating layer, however, is provided in an object quite different from that of the present invention directed to the prevention of reflection of the exposure light on the substrate surface.